The present invention relates to a line pressure control system for an automatic transmission, and more particularly to a system for controlling a servo activating hydraulic fluid pressure during shifting between different gear positions.
An automatic transmission of the RE4R01A type, manufactured by Nissan Motor Company Limited, is known. This known automatic transmission is described in "NISSAN FULL-RANGE AUTOMATIC TRANSMISSION RE4R01A TYPE, SERVICE MANUAL, (A261C07)" issued on Mar., 1987 by NISSAN MOTOR COMPANY LIMITED, and also disclosed in U.S. Pat. No. 4,680,992 issued to Hayasaki et al. on July 21, 1987. This United States Patent corresponds to European Patent Application No. 86 110 832.2 (publication number: 0214467). This automatic transmission comprises an accumulator having an accumulator piston for defining an accumulator chamber which forms a part of a hydraulic circuit leading to a frictional device and an accumulator back-up pressure chamber. During non-shifting mode when there is no supply of servo activating hydraulic fluid pressure to the frictional device, the accumulator piston is urged by the accumulator back-up pressure against an accumulator spring to take a position where the accumulator spring is compressed. The hydraulic circuit includes a shift valve and starts supplying a servo activating hydraulic fluid to the frictional device when the shift valve shifts to effect a predetermined shift between gear pisitions. Since the accumulator chamber forms the part of the hydraulic circuit, the hydraulic pressure in the accumulator chamber increases, tending to urge the accumulator piston against the bias force due to the back-up pressure. However, the accumulator piston stays in this position until the sum of the forces due to the accumulator spring and the servo activating hydraulic fluid overcomes the opposed force due to the accumulator back-up pressure. Under this condition, the servo activating hydraulic fluid pressure supplied to the frictional device increases quickly. When the above-mentioned sum of the forces exceeds the opposed force, the accumulator piston begins to move against the opposed force due to the accumulator back-up pressure in a direction to expands the accumulator spring. During this movement of the accumulator piston, the servo activating hydraulic fluid pressure increases further but at a gradual rate. When the accumulator piston moves to another limit position, the servo activating hydraulic fluid pressure further increases quickly to the level as high as the level of a system fluid pressure, i.e., a so-called line pressure. The pressure increase during the stroking of the accumulator piston is often called as a shelf portion of the pressure increase. With this pressure increase at the shelf portion, the engagement within the frictional device progresses without causing any substantial shock. The shelf portion pressure is determined by the accumulator back-up pressure, a ratio of a pressure acting area of the accumulator piston exposed to the servo activating hydraulic pressure to a pressure acting area exposed to the accumulator back-up pressure, and a preset load and a spring constant of the accumulator spring. Thus, with the same magnitude of the accumulator back-up pressure, the gradient of the shelf pressure becomes always constant. The accumulator back-up pressure is generated by an accumulator control valve controlled by a line pressure control solenoid. The accumulator control valve uses the line pressure as a base pressure and effects pressure regulation in response to a signal pressure controlled by the solenoid valve to give the accumulator back-up pressure variable with the signal pressure. During the non-shifting mode, the line pressure solenoid is controlled in response to the magnitude of a load on the engine which is expressed in terms of a throttle opening degree of the engine throttle valve by retrieving a data table prepared for the non-shifting mode based on the throttle opening degree. Another data table is prepared for the shifting mode, and the line pressure solenoid is controlled in response to the throttle opening degree by retrieving this other data table based on the throttle opening degree such that the level of the accumulator back pressure varies in proportion to the throttle opening degree. Thus, the level of the shelf pressure increases as the engine load increases even though the gradient of the shelf pressure remains substantially unchanged.
With the accumulator back-up pressure kept constant, the gradient of the shelf pressure becomes large when a spring with a large spring force (namely, a large set load) is selected as the accumulator spring, while the gradient becomes small when a spring with a small spring force is selected as the accumulator spring. During shifting with a large throttle opening degree, in order to place emphasis on endurability of the frictional device, the shelf pressure with the large gradient is preferred to shorten the time required for the shift. However, this causes substantial shock to occur during shifting with a small throttle opening. During the shift with a small throttle opening degree, the shelf pressure with small gradient is preferred for lowering shift shock to a satisfactory level. However, this causes a reduction in endurability of the frictional device since the time required for shift becomes long as compared to an appropriate time required for shift with large throttle opening degree.
Thus, it is very difficult to select a spring which can compromise both of the above-mentioned requirements with each other.
An object of the present invention is to improve the existing automatic transmission such that above-mentioned problem is solved without causing any substantial change on component parts.